[ELF] Set up initial live symbol(s) to avoid incorrect reclaim of atoms.

If no initial live symbols are set up, and deadStrip() == true,
the Resolver ends up reclaiming all the symbols that aren't absolute. This is wrong.
This patch fixes the issue by setting entrySymbolName() as live, and this allows
us to self-host lld when --gc-sections is enabled. There are still quite a few problems
with --gc-sections (test failures), so the option can't be enabled by default.

Differential Revision:	D7926
Reviewed by:	ruiu, shankarke

llvm-svn: 230737

lld/lib/ReaderWriter/ELF/ELFLinkingContext.cpp

@@ -106,6 +106,10 @@ bool ELFLinkingContext::validateImpl(raw_ostream &diagnostics) {
     _writer = createWriterELF(this->targetHandler());
     break;
   }
+
+  // If -dead_strip, set up initial live symbols.
+  if (deadStrip())
+    addDeadStripRoot(entrySymbolName());
   return true;
 }